Formation of rodlike silica aggregates directed by adsorbed thermoresponsive polymer chains.

This study deals with the fine-tuning of the interactions between silica nanoparticles and a LCST polymer in order to build permanent rigid linear aggregates. LCST polymers become hydrophobic and collapse above a critical temperature. The collapse of the polymer chains at the surface of the silica particles generates an attractive potential that can overcome the repulsive electrostatic forces between the silica particles under certain circumstances. The combined use of the thermoresponsiveness of poly(ethylene oxide) and of the chemical condensation properties of silica enables us to build permanent rigid aggregates displaying rodlike shapes just by increasing the temperature. These aggregates have been characterized using two complementary techniques: transmission electron microscopy and small angle neutron scattering. For low curing time, it appears that small linear aggregates are obtained when the electrostatic surface potential (pH = 8.5) is high and the initial ionic strength is low (I approximately = 10(-3) M). For higher heating time these objects aggregate further leading to some branching and ultimately to 3D gels which phase separate.

Responsive hybrid self-assemblies in aqueous media.

Responsive copolymers have been prepared by grafting onto a poly(acrylamide-co-sodium acrylate) backbone [PAM-co-PANa] poly(N-isopropylacrylamide) stickers [PNIPA] characterized by a lower critical solution temperature (LCST) in water. From adsorption isotherms and DSC studies performed on PNIPA/silica mixtures, it was shown that PNIPA chains irreversibly interact with silica particles and that at low coverage they partially lose their responsiveness with temperature. When PNIPA is grafted onto a PAM-co-PANa backbone, which has no specific attraction to silica surfaces (only electrostatic repulsions), their binding process remains very similar to the one analyzed with PNIPA chains alone. Above critical copolymer and silica concentrations (Cp congruent with 1 g/L and CSi congruent with 30 g/L), hybrid networks can be formed following the rules of percolation theory. The viscoelastic properties of these networks are controlled by the concentration of inorganic cross links and the fraction of PNIPA grafts participating in bridges between particles, the others being involved in inelastic loops or pendant chains. For all of the mixtures investigated, an optimum weight ratio of RSi/PNIPA = 10-15 was found for the viscoelastic properties, in agreement with the saturation of silica beads by the copolymer. Because of the responsive behavior of PNIPA in aqueous solutions, graft copolymers are able to self-assemble with temperature, giving rise to a sol/gel transition upon heating. In the presence of added silica, hybrid aggregates (silica/PNIPA) coexist at high temperature with organic ones (PNIPA/PNIPA) with synergistic or antagonistic effects on the elastic properties depending on the proportion of PNIPA grafts per silica particle.

Characterizations and properties of hairy latex particles.

Industrial latex composed of a hydrophobic core surrounded by a charged hydrophilic layer exhibits excellent stability toward monovalent salt. That feature is classically attributed to a steric effect due to a loss of entropy during overlapping of coating materials. The so-called electrosteric stabilization is, however, not a straightforward function of the nature of the hydrophilic corona. This suspension was characterized in dilute solution by scattering and electrophoresis techniques. In contrast to spherical brushes the interface between the core and the corona is not well defined. The layer is more similar to a highly hydrated nonuniform gel with few longer strands that control the hydrodynamic properties than to a polyelectrolyte brush whose dependence on ionic strength reflects the concentration of counterions inside a well-defined structure. Thus the steric contribution to stabilization of these hairy particles appears to be insignificant in the range studied. The highly hydrated nature and the global charge of the layer are two predominant factors for the stability of the particles.

Interactions of hairy latex particles with cationic copolymers.

Interactions between polycations and core-corona particles are governed by ion-exchange reactions, entropically favored by the release of counterions. This complexation process allows the chains to penetrate into the shell, leading to adsorbed amounts greater than 1 mg m(-2). The destabilization occurs quickly, the domain of flocculation becomes larger when the concentration of monovalent salts is increased, and aggregates are composed of small and very compact clusters in a more or less self-similar structure at large scale. The adsorption of copolymers of low cationicity is characterized by still larger adsorbed amounts and layers thicker than the radius of gyration of the macromolecules. Depending on the charge content, the enhancement of the ionic strength can either promote the destabilization of the suspension or conversely induce the desorption of the chain. In pure water the structure of the flocs is long-range ordered and it becomes more heterogeneous in ionic media.

A biomimetic motility assay provides insight into the mechanism of actin-based motility.

Abiomimetic motility assay is used to analyze the mechanism of force production by site-directed polymerization of actin. Polystyrene microspheres, functionalized in a controlled fashion by the N-WASP protein, the ubiquitous activator of Arp2/3 complex, undergo actin-based propulsion in a medium that consists of five pure proteins. We have analyzed the dependence of velocity on N-WASP surface density, on the concentration of capping protein, and on external force. Movement was not slowed down by increasing the diameter of the beads (0.2 to 3 microm) nor by increasing the viscosity of the medium by 10(5)-fold. This important result shows that forces due to actin polymerization are balanced by internal forces due to transient attachment of filament ends at the surface. These forces are greater than the viscous drag. Using Alexa488-labeled Arp2/3, we show that Arp2/3 is incorporated in the actin tail like G-actin by barbed end branching of filaments at the bead surface, not by side branching, and that filaments are more densely branched upon increasing gelsolin concentration. These data support models in which the rates of filament branching and capping control velocity, and autocatalytic branching of filament ends, rather than filament nucleation, occurs at the particle surface.